Having opened the drawing, I think that you have been scaling in Paper space.
In model space, the scale bars measure as quoted, i.e. your model space units are metres.
When you go into paper space, you have to remember that AutoCAD defaults to inches and millimetre units. So to scale for paper space you must divide the drawing units in millimetres by the scale required. So one metre is 1000 millimetres, then divide that by 400 which equals 2.5.
If you use Properties, and in Paper space click on the viewport, the scale of the viewport is revealed as 2.402468, so you have been altering things. Again in Properties, just correct the Custom Scale to 2.5 and that is it. Now the A3 sheet will plot out at 1 to 400, but be warned, your drawing frame has a 6mm margin all round, so you must have borderless printing on or you will not get a plot with a surrounding frame line.